Polycrystalline diamond emitter

ABSTRACT

Ultrasharp diamond edges and points which are usable as high intensity point sources for the emission of electrons, ions, X-rays, coherent and incoherent light and high frequency electromagnetic radiation are produced by preparing and classifying ultrafine diamond powder having a particle size of 10 to 100 angstroms, placing the powder in a diamond mold defining the ultrasharp edge or point to be produced and applying a pressure of the order of 80 to 90 kb while heating the powder to a temperature of the order of 2440° K. in an ultrahigh vacuum or inert atmosphere after degasing to avoid oxidation of the diamond powder.

REFERENCE TO PRIOR APPLICATION

The present application is a continuation-in-part of my application Ser.No. 608,260 filed Aug. 27, 1975 and now U.S. Pat. No. 4,084,942, issuedMay 9, 1978.

FIELD OF INVENTION

The present invention relates to the production of ultrasharp diamondedges and points which are usable as high intensity point sources forthe emission of electrons, ions, X-rays, coherent and incoherent lightand high frequency electromagnetic radiation.

BACKGROUND OF THE INVENTION

As disclosed in my U.S. Pat. No. 3,646,841, ultrasharp diamond edges arehighly useful for molecular and submolecular sectioning at ultralowtemperature and as high intensity point sources for the emission ofelectrons, ions and neutrons. Such ultrasharp diamond edges--commonlyreferred to as "diamond knives"--have also found other uses. For examplediamond knives are used in surgery particularly in delicate operationssuch as operations on the eye. It has also been found that metals andother materials can be machined with a diamond knife in such manner asto obtain a perfectly smooth surface which is free of the scratches andother imperfections resulting from usual machining and polishingoperations.

When used as point sources, diamond edges and points are coated within athin film of a suitable material such as tungsten, rhenium, lanthanum,barium, caesium and other related materials of suitable work functionand physical properties for electron emission. When thus coated, thediamond tip or edge serves as an effective emitter for electrons, ions,X-rays neutrons and other types of emission.

However, the use of ultrasharp diamond edges and points has been limitedby the cost of production and by limitations on the size of diamondedges that could be made. As high quality diamond knives are made fromdiamonds of gem quality, the cost of such knives has accordingly beenhigh. Moreover, by reason of larger diamonds not being available at aneconomically acceptable price, the length of the cutting edge of adiamond knife has been limited to less than ten millimeters and isusually in the one to three millimeter range. With respect to the use ofdiamond points and edges as cathodes or other emission electrodes, theuseful life of the electrode has been limited by the dissipation of themetallic coating material. When such material has been depleted, it isnecessary to discard or recoat the electrode. As such electrodes arenormally hermetically sealed in an appropriate envelope, the removal,recoating, remounting and resealing of the electrodes may not beeconomically feasible.

SUMMARY OF THE INVENTION

It is an object of the present invention further to extend the use ofultrasharp diamond edges and points by making it possible greatly toreduce the cost of such instruments and also to remove the presentlimitations on size. Moreover, the invention is directed greatly toextending the useful life of ultrafine diamond edges and points used ascathodes or other emitters.

In accordance with the invention ultrasharp diamond edges and points areproduced from ultrafine diamond powder by molding and bonding the powderat high temperatures and pressures in an ultrahigh vacuum or an inertatmosphere such as helium. In this manner an implement of any desiredsize and shape can be produced. Moreover, when the ultrasharp edge orpoint is to be used for electron or other emission, low work function orother emissive material in finely powdered form can be mixed with thediamond powder so as to be incorporated in the body of the electrodeinstead of being applied as a surface coating. The useful life of theelectrode can thereby be greatly increased.

In order to obtain an ultrasharp edge having a radius of 3 to 100angstrom units, the diamond powder from which the implement is moldedmust have a comparable particle size of 3 to 100 angstroms. Diamondpowder of a particle size in this range can be obtained by crushing,grinding, explosion, electric arc or thermal shock followed byclassification for example by ultrahigh speed centrifuging to separateout the finer particles whereupon oversized particles are furtherprocessed.

The bonding of the ultrafine diamond particles to form a molded articlehaving an ultrasharp edge or point is achieved either with or without abinder. For example, ultrafine nickel, chromium, titanium, niobium,zirconium, molybdenum, tungsten, copper, beryllium metal particles ortheir oxides, may be mixed with the graded ultrafine diamond powder inappropriate amounts (e.g., 10% to 40% by weight) to serve as bonding andcatalytic solvent agents. Alternatively, individual particles of thediamond powder can be coated with a suitable metal such as chromium ornickel which acts as a matrix for the diamond particles when the articleis molded. Other suitable bonding agents are LaB₄ and LaB₆.Alternatively, it is found that by using suitable pressure andtemperature in an ultrahigh vacuum or in an inert atmosphere such ashelium, bonding of the ultrafine diamond particles can be achievedwithout the use of additives.

BRIEF DESCRIPTION OF DRAWING

The objects and advantages of the invention will be more fullyunderstood from the following description by way of example of a processfor producing ultrafine diamond edges and points of the resultingproduct. The single FIGURE of drawings illustrates schematically and incross section apparatus used for carrying out the described process.

DESCRIPTION OF PREFERRED EMBODIMENTS

The process of preparing the emitter in accordance with the presentinvention comprises preparation of ultrafine diamond powder and the highpressure and high temperature molding of the product with or withoutadditives to form an implement having an ultrasharp edge or point. Theterm "implement" is herein used in a generic sense to denote a knife,cutting tool, electrode, emitter or other article having an ultrasharpedge or point.

Preparation of Powder

The ultrafine diamond powder used in the process of the presentinvention can be prepared from natural or artificial diamonds of anysize. Since the size of the implement to be produced is not dependent onthe size of the diamonds it is no longer necessary to use large gemquality diamonds. The cost of the starting material is thereby greatlyreduced. One type of diamond that is of interest for certain applicationis the type IIB semiconductive diamond discovered in 1962. Althoughdiamonds are ordinarily considered to be insulators of high dielectricstrength, it has been found that type IIB diamonds are semiconductors.The presence of aluminium and other impurities are apparently related tothis unusual property. A relatively plentiful source of this rare typeIIB diamond useful for carrying out the process of the present inventionis provided by the occurrence of this material in the extensive depositsof natural carbonado and related diamonds found in Venezuela.

The ultrafine diamond powder is suitably prepared from the natural orartificial diamonds by crushing, grinding, mechanical shock as byexplosives, an electric arc or thermal shock. The powder thus obtainedis classified for example by differential and density-gradientultracentrifugation of an oil or preferably glycerol suspension of thepowder, for example at 60,000 to 100,000 ×g. By such centrifuging,powder of the requisite fineness, for example a particle size of 3 to100 angstroms is separated out whereupon particles of larger size arereprocessed until the desired size is obtained.

Orientation of Diamond Particles In Mold

A diamond has a 1-1-1 or tetrahedral face, a 1-0-1 face as viewededgewise and a 1-0-0 face as viewed from above. The 1-1-1 face is thehardest. Hence, to obtain an ultrasharp edge or point it is desirable toorient the diamond particles in the mold so as to present the 1-1-1faces at the edge or point of the implement. Such orientation can beeffected with a strong magnetic field, a strong electric field or acombination of magnetic and electric fields. This orientation isconsiderably enhanced when the ultrafine diamond particles or plateletsare first coated (by coating with an atomic thin film evaporated in ahigh vacuum) with nickel, cobalt, iron and other ferromagnetic metalsand alloys.

Additives For Bonding

By the use of suitable temperatures, pressures and operative proceduresas herein described, the ultrafine diamond particles can be bonded toform a molded implement without the use of an additive as a bonding orbinding agent. The diamond particles have the shape of shingles or chipswith edges staggered like stairsteps. The chips have a thickness of forexample 6 angstrom and a length of 10 to 50 or up to 100 angstroms. Whenpressed together at high temperature and pressure, these chips or laminainterlock with one another to form a solid body. This is in contrastwith graphite particles which slip on one another. Thus, even withoutthe addition of a bonding or binding agent, the ultrafine diamond powdercan be effectively bonded.

However, in some instances it is desirable to use additives further toimplement bonding. Such additives should have the characteristics ofbeing hard, inert and stable and having a suitable melting or softeningtemperature so as to assist in bonding the diamond particles duringmolding. Suitable additives are alumina and especially lanthanum borates(LaB₄ and LaB₆). It is also desirable in some instances to coat theindividual particles of diamond powder with a metal, for examplechromium or nickel. The thickness of the coating should not exceed 100angstroms and preferably should be of the order of 10 to 50 angstroms.When the ultrafine diamond powder thus coated is molded at the pressuresand temperatures and under the conditions described below, the metalcoating acts as a matrix and contributes to the bonding of theparticles. The coating of the particles is conveniently effectedelectrolytically or by vaporization of the metal.

Additives For Electrical Properties

When the implement produced in accordance with the present invention isintended for use as an electrode or where electrical conductivity isdesired, suitable additives are included to provide the desiredelectrical properties. For example the additive may be one or more ofthe rare metals including the alkaline-earth metals (barium, calcium andstrontium); beryllium, bismuth, cadmium, cobalt, gallium, germanium,hafnium, indium, lithium, boron, silicon, manganese, molybdenum,rhenium, selenium, tantalum, niobium, tellurium, thallium, thorium,titanium, tungsten, uranium, vanadium, zirconium, and the rare earths.In particular, such inclusions may include tungsten, rhenium, lanthanum,barium, caesium and similar materials of suitable work function andphysical properties for electron emission. The additive can be in theform of an ultrafine powder which is mixed with the diamond powder or itcan be coated on individual particles of the diamond powder as describedabove with respect to chromium and nickel. The additive for providingdesired electrical properties is preferably in combination with nickel,cobalt or other magnetic material which is coated on the individualdiamond particles to facilitate orientation of the diamond particles inthe mold. In some instances it may be desirable to have the additiveonly in edge, point or surface portions of the implement. In this event,when the material is being placed in the mold, diamond powder containingthe desired additive is positioned at the desired location in the moldwhile diamond powder without such additive is positioned in other moldlocations. If desired, different additives of different properties canbe located in different portions of the mold and thus be present incorresponding portions of the molded implement.

Molding Of Implement With Ultrasharp Edge Or Point

In order to obtain a bonding of the ultrafine diamond powder inaccordance with the present invention, the implement to be produced ismolded at high temperature and high pressure. Thus, for example thepressure used is at least 50 kb and preferably of the order of 85 to 90kb. The temperature used is at least 2000° K. and preferably of theorder of 2440° K. to 2500° K. The temperature used may vary somewhataccording to the molding time. For example, with a pressure of 85 to 90kb, a temperature of 2440° K. is sufficient when the molding time is twominutes whereas it is desirable to use a temperature of 2500° K. whenthe molding time is reduced to one minute.

Moreover, the high temperature high pressure molding must be effectedunder conditions under which the diamond powder cannot oxidize. Thus,the molding is carried out in an ultra-high vacuum which avoidsoxidation of the diamond powder and also serves as a heat seal. Thevacuum should be of the order of 10⁻⁸ to 10⁻⁹ Torr. Alternatively, thehigh temperature high pressure molding can be carried out in an inertatmosphere for example helium after the diamond powder and itsenvironment have been thoroughly degassed in order to eliminate thepresence of oxygen.

The mold used in the process must be formed of material having thestrength, hardness and temperature resistance to withstand the stressesand temperature imposed on it during the molding operation. Moreover,the female mold must have the ability of defining on the moldedimplement an edge or point having the requitsite sharpness, e.g., aradius of 10 to 100 angstroms. In order to meet these requirements, bothportions of the mold are preferably formed of diamond which may beeither natural diamond when size permits or a molded diamond structureproduced in accordance with the present invention. Alternatively, themold can be formed of special ceramics based on ultrafine (10-50A)particles of aluminum oxide, zirconium oxide or tungstate in anappropriate filler, for example appropriate epoxy plastics. To avoid themolded article sticking to the mold, a suitable lubricant or separatingmaterial is used. For example, this can be sodium chloride or graphite.

Apparatus for Carrying out Process of Molding

In the accompanying drawing there is shown schematically moldingapparatus for carrying out the molding operation in accordance with thepresent invention. The apparatus is shown as comprising a hollowrectangular frame 1 of high strength material, for example high tensilesteel. On the base portion of the frame 1 there is positioned a moldholder 2 which may likewise be formed of high tensile steel and isprovided with a central cavity to receive a female mold 3 formed of adiamond, molded diamond composition or other suitable material. Withinthe cavity of the mold holder and superposed on the diamond mold, thereis provided a collar 4 of hard high strength heat resistant material,for example high tensile steel or tungsten carbide. A second mold holder5 likewise formed of high tensile steel is provided with a centralcavity to receive a male mold 6 likewise formed of natural or moldeddiamond or other hard material. The diamond powder 7 which is to bemolded is confined in a mold cavity defined by the female mold 3, theretaining collar 4 and the male mold 6.

Means is provided for heating the powder to be molded to the requiredtemperature (2440° K. to 2500° K.) during molding. The heating means isshown schematically in the drawings as comprising an electricalresistance heating element 8 surrounding the mold and mold holders. Forexample, the heating element may comprise a strip of Kantal, rhenium,tungsten or other suitable resistance material. The electricalresistance heating element 8 can surround the mold, as shown, or can beplaced directly within the mold, close to a precision thermocoupleconnected through a suitable amplifier-recorder-feedback circuit to theheater power supply in order to maintain the requisite temperatureduring the high pressure cycle. The heating element 8 is shownelectrically insulated on both sides by layers 9 and 10 of suitableinsulating material such as pyrophyllite or lithographic stone. Heatinsulation is provided by a thick layer 11 of suitable thermalinsulating material such as, pyrophyllite, asbestos or lithographicstone which surrounds the heating element with its electricalinsulation. The heating element and electrical and thermal insulationare assembled within a retaining sleeve 12 formed for example of steel.

Instead of heating conductively as shown, heating to the requiredtemperature may be effected by other means, for example by inductionheating, microwave radio frequency heat, a laser beam or an electronbeam. The ultrahigh vacuum under which the heating is effectedfacilitates the use of laser or electron beam heating introduced throughdiamond windows provided in suitable oriented apertures. Alternatively,a cooled copper target bonded to diamond heat sinks (of type IIadiamond) may be used in conjunction with suitably placed electron beamguns embedded in a heat refractory and insulating matrix close to themold.

Suitable means is provided for applying the required pressure (85 to 90kb) during molding. As illustrated by way of example in the drawing, thepressure applying means comprises a superconductive electromagnetcomprising a superconductive winding 14 and armature 15 formed forexample of soft iron or other material having suitable magneticproperties. Superconductivity of the winding is achieved through the useof ultralow temperatures. By virtue of the superconductivity of thewinding, the forces required to achieve the desired pressure can beachieved. Suitable cryogenic assemblies and suitable superinsulation(not shown) are provided for maintaining the requisite liquid heliumtemperatures (4.2° K.) essential for the superconductive solenoid. Otherelectromagnetic systems may also be used for producing the ultrahighpressures required in the process of the present invention.

The molding press as thus described is mounted on a steel base 16 and isenclosed by a bell jar or other envelope 17 which seats on the base witha hermetic seal 18. The bell jar 17 can be made of tempered glass but ispreferably made of stainless steel fitted with heat resistantvacuum-tight ports or windows arranged for observing the entire processwith light microscopes, scanning electron microscopes, X-ray diffractionand X-ray scattering techniques, thermometry and mass-spectrometry fordetection of residual gasses. A high vacuum pump is indicatedschematically at 19 for producing an ultrahigh vacuum within theenvelope 17. If after evacuation to achieve through degasification it isdesired to provide in the enclosure an inert atmosphere such as helium,a suitable supply and suitable connections (not shown) are provided.Likewise, suitable elctrical connections are provided for the heatingelement 8 and the superconductive electromagnet winding 14. Preferablyone or more suitable thermocouples are provided for sensing thetemperature of the mold.

The female mold 3 is formed in such manner as to provide a cavity whichdefines on the molded implement an edge or point of the requiredsharpness. One method of achieving this is to form the mold cavity in anatural diamond or in natural carbonado with a laser beam.Alternatively, a block of molded diamond composition made in accordancewith the process of the present invention can be used as a mold body inwhich the cavity is thus formed. Alternatively, the female mold can beformed by the powdered diamond technique as herein described with theuse of natural or molded diamond knife to produce the mold cavity duringthe molding operation.

Molding Operation

Use of apparatus shown in the drawings for carrying out the process ofthe present invention will be readily understood from the foregoingdescription. The diamond powder to be molded together with any additivesis placed in the cavity of the female mold 3 in such a way, e.g., usingelectrophoresis or catophoresis, as to make sure that the particles getdown into the submicroscopic mold edge by the passage of an electriccurrent in the presence of a magnetic field of suitable strength andconfiguration and are properly oriented. The filled and assembled moldis positioned together with heating element and pressure applying meansin the frame 1. The envelope 17 is closed and the space within theenvelope is evacuated to achieve an ultrahigh vacuum (10⁻⁸ to 10⁻⁹Torr.) and to thoroughly degas the diamond powder and the entireassembly inside the envelope. The required temperature and pressure arethereupon applied to effect a bonding of the powder and molding of thedesired implement. When using a superconducting magnetic coil to providethe required pressure as illustrated in the drawng, the magnet isenergized to effect and maintain orientation of the diamond particles.The magnetic field is built up gradually and then suddenly increased toproduce a magnetic shock to effect the final molding and setting of thepolycrystalline diamond implement. The magnetic field used in the rangeof 50,000 to 200,000 gauss.

As indicated above, the diamond particles in the mold are preferablyoriented to present the 1-1-1 faces at the edge or point of theimplement being molded. If it is desired to have certain additives onlyin certain portions of the implement the powder containing suchadditives is suitably positioned in the mold. Moreover, a metal insertor a lead for electrical connection can be inserted in the mold asdesired.

Implement Produced According To The Invention

As will be understood from the foregoing explanation of the process, animplement formed in accordance with the invention has an ultrasharp edgeor point which is defined by the mold in the molding operation.Subsequent finishing or polishing not only is unnecessary but is highlyundesirable as it would disrupt the extremely fine edge achieved by theprocess of the present invention. Such edge has a sharpness defined by aradius which measures only 3 to 100 angstroms and is straight anduniform as in the case of a natural diamond knife as described in U.S.Pat. No. 3,646,841. However, in contrast with the naturalmonocrystalline diamond knife, a polycrystalline diamond implement madein accordance with the process of the present invention can be of anydesired length.

If the implement is to be used for electrical purposes for example forthe emission of electrons, ions, neutrons, X-rays, coherent orincoherent light and high frequency electromagnetic radiation, it isprovided with inclusions of suitable material for example tungsten,rhenium, lanthanum, barium, caesium etc. as described above. Suchinclusion may be throughout the implement or may be limited to localportions, for example the edge portion or a surface layer. Likewise, asdescribed above, the molded implement may include bonding media such asalumina and lanthanum borate. When it is desired to make an electricalconnection to the molded implement, a suitable metal insert or lead canbe molded into it. It will be understood that many variations are madepossible by the process in accordance with the invention. Thus theemitter may be in the form of an edge, a point or a comb or other shapewith a plurality of points. Moreover, the point or points, instead ofprojecting, may be "negative" or female points, i.e., in the form of asharp pointed cavity or recess. The included angle of the edge or pointmay be selected as desired, for example from 30° to 60° and the sides,instead of being planar, may be curved, e.g., hollow ground, if desired.Moreover, the cost of production of implements having ultrasharp edgesor points is greatly reduced. The invention thus makes possible asignificant extension of the use of diamond tools for cutting and ofdiamond edges and points for use as emitters.

Classification of the diamond powder used in carrying out the process ofthe present invention is important for the reason that best results areobtained with a power of approximately uniform particle size. Forcritical work it is desirable to have the particle size uniform with arange of 50 angstroms and preferably 20 angstroms. For a diamond knifehaving an ultrasharp cutting edge, it is desirable to use diamond powderhaving a particle size of 3 to 20 angstroms. If the implement is to beused as an emitter, the particle size of the diamond powder can belarger for example up to 100 angstroms. By reason of the ultrafinepowder used and the pressure and temperature at which it is molded, itis possible to obtain by the process of the present invention a materialhaving a density equal to that of a natural diamond. When additives areused which have a density higher than that of diamonds, the resultingproduct has a density higher than that of a natural diamond.

Advantages And Applications Of Ultrahard Materials Produced by Processof Invention

In accordance with the present invention the features of ultrahighvacuum, high field superconducting magnets, natural or polycrystallinesynthetic diamond anvils with suitable "windows" for laser electron beamor radiation heating are all incorporated in a relatively simple andabove all incomparably clean and precise high pressure diamond anvilapparatus which lends itself ideally to constant monitoring and directobservation of critical changes in temperature, fine structure asrevealed by light microscopy, scanning electron microscopy and modifiedtransmission electron microscopy and diffraction and determination ofthe properties of the finished ultrahard diamond-composite part atmolecular and atomic levels by X-ray diffraction and scattering, laserinterferometry, spectroscopy, etc. Moreover, the apparatus illustratedby way of example in the drawings can be readily scaled up in sizemerely by increasing the dimensions and strength of the magnetic fieldelements (for example larger superconductive magnets or nitrogen cooledhigh-field Bitter-type magnets, etc.) and the size of the anvils whichcan be made progressively larger by using new types of diamond-compactsof polycrystalline type.

A significant feature of the invention that has now been clearlydemonstrated is the fact that the material produced in accordance withthe invention both in the form of a so-called "polycrystalline diamond"or "synthetic carbonado" represents a totally different type ofmaterial. This appears to result at least in part from the uniformlyultrasmall particle size (3 to 100 angstroms and high surface to volumeratio of the diamond particles which are used. Up till now all of thereported sintered diamond or ultrahard diamond compacts ofpolycrystalline diamond with suitable solvent metal catalysts oradmixtures have been made from diamond particles in the size range of 1to 100 microns and no attempt has been made to classify or grade theparticles. A cube which is 1 centimeter on a side has a total surfacearea of 6 square centimeters and a volume of 1 cubic centimeter andhence a surface area to volume ratio of 6 sq. cm/cm³. A cube which is 1micron on a side has a total surface area of 6×10⁻⁸ square centimeters,a volume of 10⁻¹² cubic centimeters and hence a surface area to volumeratio of 60,000 square centimeters/cubic centimeters. This largeincrease in surface area to volume ratio would be quite remarkable ifanyone has succeeded in accurately classifying or grading the micronsize powder and sintering it at high pressures and temperatures withoutoxidation. This is still an unattained goal. In the process of thepresent invention we are dealing with particles of about 0.01 micron or100 angstroms in side dimensions and even though the particles are inthe form of a more favorable platelet instead of a cube, the particleshave a total surface area of the order of 6×10⁻¹² square centimeters,and a volume of 10⁻¹⁸ cubic centimeters with a surface area to volumeratio of the order of 6,000,000 square centimeters/cubic centimeters.The material produced in accordance with the invention is apolycrystalline ultrahard sintered material made up of either uniformultrafine diamond particles in homogeneous array and in differentorientations which would give a material resembling the glassy orvitreous state but incomparably harder with highest tensile strength andlowest thermal expansion coefficient. There is thus produced a new typeof diamond glass or ceramic particularly in case of mixing ultrafinediamond particles with suitable bonding metals or catalytic solvents.There are thus achieved unique qualitative differences of this newmaterial which goes well beyond a mere quantitative difference in size.

Moreover, it has been found that the same method and apparatus can beused to make synthetic diamond in polycrystalline form by using verypure ultrafine graphite prepared by high vacuum evaporation and thensubjecting a mixture of these graphite crystallites having a particlesize of about 10 to 50 angstroms with a wide variety of ultrafinemetallic catalytic solvents such as iron, nickel, cobalt, chromium,manganese, palladium, iridium, platinum, tantalum, ruthenium, most ofwhich belong to group VIII of the periodic table or with certain binarycatalytic systems forming alloys composed typically of transitionelements such as titanium, zirconium, niobium, molybdenum, tungsten andhafnium which belong to group IVa, Va or VIa of the periodic tablecombined with elements such as copper, silver or gold or group Ib of theperiod table. The ultrafine particles of the metallic materials aresimilarly prepared by high vacuum arc evaporation or sputtering in anoble gas atmosphere. The process in accordance with the presentinvention thus permits starting with pure ultrafine graphite instead ofultrafine diamond powder and to obtain diamond in polycrystalline formby the reaction of a metallic catalytic solvent or carbon under the samehigh pressures (for example 50 to 60 kb) and high temperatures (2000° K.to 2500° K.) required to attain the region of thermodynamic stability ofdiamond.

While preferred embodiments of the invention have been particularlydescribed, it will be understood that many modifications may be made.For example other additives can be used with the ultrafine diamond orgraphite powder and two or more additives can be used in the samearticle. For example alumina or lanthanum borate can be used jointlywith a metal selected for its electron emission or other electricalproperties. When one or more additive is used, the ultrafine diamond orgraphite powder should constitute at least 50% and preferably at least60% of the composition.

What is claimed is:
 1. A polycrystalline diamond emitter having anultrasharp edge or point, said emitter comprising a multiplicity ofdiamond particles molecularly bonded together with the cohesion ofnatural diamond, all of said diamond particles being of a size less than100 angstroms and being uniform in size within a range of 50 angstroms,said diamond particles being uniformly oriented to present like faces tosaid edge or point to define a uniform edge or point having a radius offrom 3 to 100 angstroms, said emitter having at least in the portionforming said edge or point uniform inclusions of low work functionmetal.
 2. An emitter according to claim 1, in which said inclusions areselected from the group consisting of rare metals.
 3. An emitteraccording to claim 2, in which said inclusions are selected from thegroup consisting of rare-earths.
 4. An emitter according to claim 1, inwhich said inclusions are selected from the group consisting oftungsten, rhenium, lanthanum, barium and caesium.
 5. An emitteraccording to claim 1, in which said diamond particles have 1-1-1 facesand are oriented to present said 1-1-1 faces at the edge or point ofsaid emitter.
 6. An emitter according to claim 1, in which saidinclusions are in the form of coatings on individual diamond particles.7. An emitter according to claim 1, in which said inclusions are in theform of particles interspersed with said diamond particles.
 8. Anemitter according to claim 1 in which said inclusions comprise thorium.9. An emitter according to claim 1, in which an ultrafine additiveselected from the group consisting of LaB₄ and LaB₆ is intimately mixedwith said diamond particles.
 10. An emitter according to claim 1, inwhich individual diamond particles are coated with metal selected fromthe group consisting of chromium, nickel, cobalt and iron, said metalcoating acting as a matrix which contributes to the bonding of saiddiamond particles.
 11. An emitter according to claim 1, in which saiddiamond particles comprise platelets having a thickness of the order of6 angstroms and a length of 10 to 100 angstroms, said platelets beinginterlocked with one another to form a solid body.